1. Field of the Invention
This invention relates to the electrodeposition of chromium from trivalent chromium plating baths.
2. Brief Description of the Prior Art
The success of a trivalent chromium plating operation frequently depends on the prevention of interference from common metal ions such as iron, nickel, copper, zinc and lead which have entered the bath. In general, trivalent chromium plating processes are known to have very low tolerance to metallic impurities. The tolerance of the bath to zinc and copper is especially low, thus necessitating the exercising of extreme care to avoid contamination with these metals. Copper can be plated out using low current density electrolysis, but zinc does not plate out well and consequently poses a greater problem.
Typically, zinc contamination is unavoidable when the plating is carried out on brass or zinc diecastings. Zinc is extremely active and will dissolve readily under the acidic condition of the bath, and consequently, dropped parts must be removed rapidly. In the case of parts that are not completely plated prior to chromium plating, as for example due to extremely low current density areas, some dissolution is unavoidable. As noted, for example, in U.S. Pat. No. 4,093,521, zinc should be present at a level of below 20 ppm in order to avoid plating faults. At a level of 20 ppm, or greater, a whitish hazy band will appear at the lower limit of the plating range. As the zinc level increases in the bath, the coverage is reduced and the white haze moves into the higher current density areas.
The only method commonly employed for the removal of zinc, are the precipitation processes, as described for example, in U.S. Pat. No. 4,038,160. The process involves the use of a water soluble ferrocyanide to precipitate trace metal contaminants including zinc, from the bath. The process is time consuming and it is known that cathodic filming can occur. The film is so heavy that, in some instances, parts plated in a solution which employs ferrocyanide must be physically wiped down to remove the adherent powder.
Moreover, it is pointed out in the patent that an addition of ferrocyanide in amounts in excess of those required to eliminate faults may cause a deterioration in the performance of the bath. Incremental ferrocyanide addition is used in combination with observation of the deposit in order to determine the proper concentration of ferrocyanide. An excess of ferrocyanide is combated by employing additional metal such as nickel, zinc, iron or copper. The process control employs a "rule of thumb" to equate ferrocyanide concentration to contaminant concentration.
Even though the solution tolerance to nickel may be greater than to most other metals, nickel contamination is especially difficult to contend with because ordinarily, it does not readily plate out. In some instances, nickel is added to the plating bath as part of its composition, as seen for example, in U.S. Pat. No. 3,954,574. Nickel can be present to saturation for the purpose of codeposition. The level of nickel tolerance is inversely related to the required quality level of the appearance of the deposit. In this regard it is noted that the plating baths of U.S. Pat. No. 4,093,521 can only tolerate up to 150 ppm of nickel, or up to 100 ppm nickel in the presence of iron, with a combined concentration of up to 150 ppm of iron and nickel.
Nickel contamination removal commonly involves the use of precipitation with an agent such as dimethylglyoxime. The high cost of the agent and the difficulty of precipitating the resultant flocculant precipitant render this technique less than perfect. A ferrocyanide salt can also be used but not without problems, as previously noted.
It is noted that some of the compounds of the instant invention are used as additives in nickel plating. However, not all nickel brighteners are useful in trivalent chromium plating baths for increasing the baths' tolerance to nickel impurities. Surprisingly, it was found that some of these additives were also beneficial in handling zinc impurities, even though typical brighteners used in zinc plating, such as aldehydes, had no beneficial effect in trivalent chromium plating baths containing zinc contaminants.